The standard method of aiming an artillery piece such as a tank gun from a director remote from the gun entails measuring the director angle whose apex is at the director and whose one side is defined by a base line 3200.sup.- offset from the sight line extending from the director to the target and whose other side is defined by a line extending from the director to the tank, as both the target and the gun are visible from the director. This tank angle is then radioed to the tank whose turret is pivoted until the angle formed between the tank barrel and a line from the tank to the director is equal to the director angle. This turning is controlled by a scale in the tank that is set to zero at the director angle.
Such a system will produce a perfect aiming of the tank barrel parallel to the sight line if the director and tank angles are both determined from the center of rotation of the tank turret from which the barrel axis extends. Normally, however, both sightings are taken at the tank periscope which, because of space constraints, is mounted somewhat offset from the turret axis so that as a result the barrel position is slightly off. An initial angular error of 10.sup.- (0.5625.degree.) is common, whereas accurate shooting requires an angular error of at most 1.sup.-. This maladjustment is corrected by iterating the sighting procedure, taking two new readings from the director and one from the gun and repivoting the tank turret in accordance with the new readings. For very accurate shooting this iteration procedure must be done several times.